As known in the art, halogen incandescent lamps, also referred to as tungsten halogen lamps, generally resemble other types of incandescent lamps, but differ in part in that the outer glass jacket (envelope) of a halogen lamp encloses a capsule in which a light-generating element, commonly referred to as a filament, is contained. The capsule also typically contains an inert gas, for example, krypton, xenon, and/or argon, and a small amount of a gaseous halogen species, for example, a bromine compound. The halogen species achieves a halogen cycle chemical reaction within the capsule that is capable of increasing the life of the filament, enables higher operating temperatures, and can promote certain lighting characteristics as compared to other types of incandescent lamps.
Because incandescent lamps are generally less efficient than other types of lighting, for example, compact fluorescent lamps (CFL) and light-emitting diodes (LED) lamps, governing authorities have taken steps to mandate increased efficiencies for lamps. An example is the Energy Independence and Security Act (EISA) of 2007, which sets luminosity requirements within the U.S.A. for given lamp wattages and categories of lamps, effectively mandating minimum standards for energy efficiency measured in the industry on the basis of lumens per watt (LPW) of electricity input to the lamp.
A “modified spectrum” lamp is a category of general service incandescent lamps defined by the EISA. According to the EISA definition, the modified spectrum lamp is a type of incandescent lamps that is intended for general service lighting applications and not sufficiently saturated in color to be categorized as a colored incandescent lamp. FIG. 1 utilizes what is referred to in the art as the CIE (Commission Internationale d'Eclairage) 1931 color space chromaticity diagram to illustrate the requirements of the modified spectrum lamp in terms of a color space relative to a clear ANSI (American National Standards Institute) A19-type incandescent lamp that serves as a “clear center.” As used herein, a “clear center” 10 in FIG. 1 refers to the light emitted by a “clear baseline lamp” that lacks any doping such as clear A19 lamp, coating, or other treatment that alters the color of the white light emitted by the lamp filament. As known in the art, a color space is a mathematical model of how colors can be represented as values in an x-y coordinate system, and a MacAdam ellipse (oval) refers to a region in the color space in which the colors are indistinguishable by the human eye. A modified spectrum lamp must have chromaticity coordinates (CCx and CCy), as shown in FIG. 1, below a black-body locus 12 and outside the four-step MacAdam ellipse 14 of the clear baseline lamp. If a lamp meets the requirements to be considered a modified spectrum lamp, the EISA reduces its luminosity requirement for a given wattage by 25%. As such, an incandescent lamp that meets the definition of a modified spectrum lamp can provide for the possibility of a much wider range of applications and/or design possibilities because of the relatively more lenient luminosity requirements for such a lamp.
Modified spectrum lamps have been produced to have an outer jacket formed of a glass modified to filter certain wavelengths of light. A notable example is a neodymium oxide (neodymia, Nd2O3) doped glass used in the REVEAL line of incandescent bulbs commercially available from GE Lighting. A different approach is to apply a pigment-doped coating on the interior of the outer jacket. Though predating the EISA definition of the “modified spectrum” lamp, an example of such a coating is disclosed in U.S. Pat. No. 5,252,887 by Reisman and, under the existing definition, would result in a modified spectrum lamp if applied to an appropriate coating thickness. These concepts are known in the art, but are generally lacking in their ability to provide modified spectrum conditions for a broad range of incandescent lamps, both in terms of visual appearance and spectral power distribution. In addition, these approaches may be cost prohibitive or reduce the luminosity of the lamp to what may be an impractical extent.